1. Field of the Invention
The present invention relates to a printing apparatus, and more specifically to a registration adjustment of a full-line head in which printing elements such as ink ejection ports and the like are arranged corresponding to a width of a paper which is fed.
2. Description of Prior Art
As an example of a printing apparatus using a plurality of full-line heads of this kind, a full-color page printer is known which is equipped with heads for respectively ejecting four color inks of black, cyan, magenta, and yellow by utilizing generation of bubbles by heat energy (hereinafter referred to as thermal line head). An example of a structure thereof will be described below.
FIG. 1 is a block diagram showing a control arrangement of a full-color printer equipped with a thermal ink jet line head, hereinafter referred to as a thermal line head for each ink of black, cyan, magenta, and yellow having ink ejection ports in a density of 600 dpi over a range of about 8 inches corresponding to a paper width when, for example, an A4-sized print paper is used to be fed in the longitudinal direction thereof.
In the FIG., a reference numeral 1 denotes a CPU for executing operation control of the entire apparatus and data processing, a reference numeral 2 denotes a ROM for storing control programs of the CPU 1, fonts, and various data for processing, a reference numeral 3 denotes a RAM for used as a work area for processing of CPU 1 such as temporary storage of various data, a reference numeral 4 denotes a data receiving unit for making communication control with external devices (not shown) such as a host computer to receive data from the external devices, a reference numeral 5 denotes a DMA/RAM controller for performing DMA transfer of data from the data receiving portion 4 to the RAM 3, performing DMA transferring of image data stored in the RAM 3 to a line head controller 9, and controlling access of the RAM 3 by the CPU 1, and a reference numeral 6 denotes a non-volatile memory such as an EEPROM for storing parameters specific to the printer. Reference numerals 7K, 7C, 7M, and 7Y respectively denote thermal line heads for individually ejecting black (K), cyan (C), magenta (M), and yellow (Y) inks to a printing medium such as printing paper to print an image, reference numerals 8K, 8C, 8M, an 8Y denote line head drivers for individually driving the thermal line heads 7K, 7C, 7M, and 7Y, and a reference numeral 9 denotes the line head controller for performing image data transferring to the individual line head drivers 8K, 8C, 8M, an 8Y, generating of heat pulse signals, and the like according to control signals from the CPU 1.
A line feed motor driver 10 and a line feed motor 11 driven by the driver 10 perform rotation drive of a feed roller (not shown) according to control signals supplied from the CPU 1, thereby performing movement of the printing medium in a sub-scanning direction, that is, paper feed.
FIG. 2 is a block diagram showing details of the line head controller 9 and the line head driver 8. For simplicity, the FIG. shows only an arrangement for one color of ink.
In the line head controller 9 shown in FIG. 2, a reference numeral 91 denotes a heat pulse generation portion for generating pulses for driving the thermal line head in synchronization with the drive of the line feed motor, a reference numeral 92 denotes an image data latch for receiving image data stored in the RAM 3 through the DMA/RAM controller 5 (see FIG. 4), a reference numeral 93 denotes a parallel/serial converter for, to data from the image data latch 92, adding offset data relating to a beginning ejection port, that is, offset data from a reference ejection port to an end most ejection port whose data showing "ejection" in the ejection port row of the thermal line head, and thereafter performing parallel/serial conversion to serially transmit a serial data to the line head driver 8, and a reference numeral 94 denotes an offset setting register for setting the offset data relating to the beginning ejection port. On the other hand, in the line head driver 8, a reference numeral 81 denotes a serial/parallel converter for performing serial/parallel conversion for the serial data, a reference numeral 82 denotes a gate for performing OR operation between an output of the serial/parallel converter 81 and heat pulse from the heat pulse generation portion 91, and a reference numeral 83 denotes a transistor array for respectively supplying an electric current to an electro-thermal conversion element of the thermal line head according to an output of the gate 82.
Next, a control procedure in the above prior art arrangement will be described with reference to FIGS. 1 and 2.
First, data inputted through the data receiving unit 4 is temporarily stored in the RAM 3 through the DMA/RAM controller 5, and an analysis of command, image data, character code is made by the CPU 1 according to a program stored in the ROM 2. The data stored in the RAM 3 is then converted into ejection data by the CPU 1, and sequentially stored in the RAM 3. When development of the ejection data in the RAM for one page is completed or a print instruction is inputted from the host computer as the external devices, the line feed motor 11 is driven by the line feed motor driver 10 and, at the same time, the eject data stored in the RAM 3 is transferred to the serial/parallel converter 81 of the line head driver 8 through the DMA/RAM controller 5 and the parallel/serial converter 93 of the thermal line head control unit 9. When the transfer of ejection data of one line is completed and the line feed motor rotates at a predetermined amount, a heat pulse signal is sent from the heat pulse generation portion 91 of the line head controller 9 to the gate 82 of the line head driver 8, which switching drives the transistor array unit 83 to cause the electro-thermal conversion element of the corresponding ejection port of the thermal line head to generate heat energy, and the ink is ejected from the ejection port by bubbles produced thereby.
The four thermal line heads 7K, 7C, 7M, and 7Y are arranged in the sub-scanning direction which is a feed direction of the printing paper. Therefore, it is necessary that the ejection data is transferred at a timing according to the distance between the individual thermal line heads in the sub-scanning direction with respect to the individual thermal line heads so that a plurality of ink dots is overlappedly formed at the same position on the paper. For example, when the spacing in the sub-scanning direction between the individual ejection ports of the individual thermal line heads is 1 inch and the resolution of dots formed in the sub-scanning direction is 600 dpi, if yellow and magenta are to be overlapped at a position to make red, yellow is first ejected to form a dot, and after the printing paper is fed by 1 inch in the sub-scanning direction, the magenta ink ejected to form an overlapped dot. In other words, the ejection data when ejecting magenta ink at the same time of ejecting the yellow ink must be data of 600 dots (600 rasters) before.
Next, a registration adjustment in the above prior art arrangement will be described.
A registration adjustment is a procedure for reducing a deviation of deposited position of ink droplets ejected by different thermal line heads to a small value or zero, and includes an adjustment in the sub-scanning direction and an adjustment in the main scanning direction which is the arrangement direction of the ejection ports. In general, when the deviation is not less than 1/2 the dot diameter, the image quality may be degraded.
A cause of generation of deviation in the sub-scanning direction is a mounting error of the thermal line head or feeds fluctuation of printing medium. The deviation in the subscanning direction is adjusted by providing an adjustment mechanism in the mounting unit or by making ejection timing of each thermal line head variable.
On the other hand, the cause of deviation in the main scanning direction is mainly a mounting error of the thermal line head, and the deviation can be adjusted by providing an adjustment mechanism in the mounting unit to adjust a portion of the ejection port of individual heads. Further, when the printable width of the individual thermal line heads, that is, the arrangement width of the ejection ports is longer than a width required for printing, as shown in FIG. 3, an offset can be adjusted for each head with respect to the beginning ejection port, thereby correcting a mounting error between individual thermal line heads to reduce deviation of registration.
As described above, in the full-color page printer equipped with the plurality of thermal line heads, it is necessary to position the dots of individual colors as exact as possible. In the case of printing with a resolution of 600 dpi in the main scanning direction shown in the above prior art example, since a width allocated to 1 dot, that is, a dot pitch, is about 42 .mu.m, if the dot position is deviated by more than 21 .mu.m or a half of the dot pitch, the image quality is degraded.
Incidentally, an exact adjustment of relative positions of the individual thermal line heads can be performed relatively easily at a delivery from the factory. However, in a normal operation environment, temperature of the thermal line head may be changed due to a change in ambient temperature or printing operation. When thermal expansion occurs in various parts of the head due to the temperature change, the following problems may be caused. For example, in the case that a base plate of the thermal line head is made of aluminum, since the thermal expansion coefficient of aluminum is about 2.times.10.sup.-5 /.degree. C., the head length is increased at a rate of about 4 .mu.m/.degree. C. in a printer for A4-sized paper. In this case, if the individual thermal line heads are at the same temperature, since the relative positions in the main scanning direction are not changed, no problem occurs in the print quality. However, if there is a temperature difference of more than 5.2.degree. C., the above described position deviation tolerance of 21 .mu.m that can maintain the print quality is exceeded, resulting in considerable degradation in image quality.
On the other hand, it is very difficult to keep the individual thermal line heads within such a narrow temperature range, and in order to achieve this, a large scale temperature control apparatus must be provided, which may lead to an increase in cost.